Our overall objective is to characterize the cell surface membrane complex, using flourescence spectroscopy and microscopy in conjunction with detailed cytological, biochemical, and functional analyses. The proposed experiments are designed to test our previous hypothesis that there is a cell surface modulating assembly that can control the mobility of surface receptors and that can influence cell growth, motility, and interaction with other cells. In particular, we will use fluorescence photobleaching recovery and biochemical methods to measure quantitatively the mobility of cell surface receptors and to identify cytoplasmic components that may interact with surface receptors to form such a modulating assembly.